The present invention relates to an electrical discharge milling machine using tool electrodes of elongate shape and of constant cross section, said tools carrying out the machining at their ends and being subject to longitudinal wear,
said machine being equipped with:
                (a) an erosive discharge generator,        (b) a rotating spindle with a mandrel,        (c) a numerical controller (NC),        (d) a set of motorized axes controlled by the numerical controller,        (e) a device for measuring the length of the tool outside the machining process,        (f) a tool path generator (CAD/CAM system) interfaced with the numerical controller, which can be used to divide the workpiece into a stack of successive layers and define one or more paths for the sweep of the tool in each layer, and        (g) a post-processor acting as a set value generator, interfaced with the numerical controller or integrated therein.        
The tools normally used for electrical discharge milling are tubes, or less commonly cylinders. However, because the tool is made to rotate, it is possible to use any type of tool having a constant cross section, including, for example, a square or rectangular cross section.
The electrical discharge milling method is a well-known variant of EDM machining, documented in depth in the thesis by Philip Bleys, “Electrical Discharge Milling: technology and tool wear compensation”, Université Catholique de Louvain, December 2003.
To achieve acceptable precision in the machined workpiece, the electrical discharge milling method requires continuous compensation for the wear of the electrode tool. Given that the shape of the tool end is normally unvarying, its wear can be compensated for in a single dimension, in other words incrementally along its axis of symmetry which is generally identical to the Z axis of the machine.
A tested method is that of including compensation commands in the program which describes the tool path. The term “anticipated corrections” is generally used in this case, as the corrections are determined in advance. These corrections are made in the form of an inclination or gradient of the tool path, such that the tool penetrates into the workpiece as it advances on its path. This method of anticipated compensation is supplemented by periodic measurements of the actual length of the tool outside the machining process, using a reference sensor; by this means it is possible to measure the length of the tool outside the machining process, to check that the programmed wear matches the actual measured reduction in length, and to make corrections.
The machine is connected to a CAD/CAM system which is a program having the function of dividing the workpiece into superimposed layers and generating tool paths for the numerical controller (NC) at each layer. Between the CAD/CAM system and the NC it is generally necessary to implement a post-processor program having the function of introducing into the machining program the appropriate technological parameters for the electrical discharge milling machine, together with the nominal or limit speeds of tool advance. In some embodiments, the post-processor can be incorporated into the NC. On the basis of the information from the CAD/CAM system, the post-processor executes a preliminary simulation of the machining process, in which, notably, the points at which the material has not yet been machined are identified, the volumes of material still to be machined are predicted, and the corresponding wear on the tool is calculated according to a more or less precise model.
This method is subject to a drawback to which Bleys proposes a solution; see, notably, Figure 6.52 of Section 6.9.3 of the document cited above. The unmachined workpiece must be described by a very precise geometrical model, using the CAD/CAM program. Bleys proposes a device for stopping the wear compensation when the tool enters a void in the material which is not identified in the CAD/CAM program.
The original combined compensation device developed by Bleys includes a first “real time” branch and a second “anticipated” branch. When the real time branch is activated, it counts the effective discharges occurring within a time interval sequenced by a clock, and immediately applies wear compensation to the tool in proportion to the recorded number.
The anticipated branch makes available a sequence of compensation gradients determined according to a program. These gradients can be applied in succession to precise curvilinear abscissas defined in advance along the whole path.
Using the combined method of Bleys, the calculations are performed simultaneously in both the real time and the anticipated branch, but only one of the two generates the compensation command, on the basis of a comparison of the two results.
It is desirable for the real time branch to generate compensation commands continuously. The real time branch does not hand over control to the anticipated branch unless the former sends a higher compensation command than the second. The real time branch serves, notably, to detect movements of the tool in voids in the material which were not identified in advance, and to stop the compensation. The real time branch is the improvement proposed by Bleys which makes it possible, notably, to detect movements of the tool in voids in the material which were not identified in advance, and to stop the compensation. However, this entails a constant risk of drift if the system allows the smallest difference between the measurement of the wear and the compensation for it. To reduce this risk, the anticipated branch acts as a safety barrier and prevents the accidental downward drifts that are intrinsic to the real time branch.
In the Bleys system, therefore, we find a real time branch having temporal sequencing and an anticipated branch having spatial sequencing; the wear correction is directly injected at the Z axis which is also sequenced by a clock.
However, this method has a drawback, in that, if the wear parameters are selected incorrectly, the system may become blocked in the anticipated branch, because the two branches operate according to an evaluation of the wear on the tool and it is known that this wear can vary dramatically according to the machining conditions. See Bleys, Section 4.6. A solution to this problem has been proposed in the patent EP1238740B1, but the method described therein is complicated and costly. Consequently the drawbacks due to the variability of the wear on the tool persist.
On the other hand, Figure 3.18 of Section 3.5.2 of the Bleys document illustrates another problem which has not been satisfactorily resolved in the case in which the geometrical model of the unmachined workpiece has not been fully worked out in the CAD/CAM program. This problem concerns the risk of cumulative deformation of the end of the tool beyond the thickness of the layer, in a case where it has insufficient, but not zero, engagement with the material in a prolonged stroke; in this case, the overlap ZL between the workpiece and the tool is too small and the wear on the tool ceases to be uniformly distributed over its radius.